This application further claims priority under 35 U.S.C. xc2xa7 119 of Swedish National Application No. 0103006-3, filed Sep. 10, 2001, entitled Homogenization of a Spatially Coherent Radiation Beam and Reading/Writing of a Pattern on a Workpiece.
The present invention relates in general to homogenization of the spatial intensity distributions of spatially coherent radiation beams. It also relates to high precision printing of patterns on photosensitive surfaces of workpieces, such as photomasks for semiconductor devices and displays. The invention also applies to illumination of workpieces for reading back a pattern for inspection of patterns or surface defects.
It is known in the current art to build precision pattern generators using projection of micromirror spatial light modulators (SLM""s) of the micromirror type (Nelson 1988, Kuck 1990). The use of an SLM in a pattern generator has a number of advantages compared to the more wide-spread method of using scanning laser spots: the SLM is a massively parallel device and the number of pixels that can be written per second is extremely high.
Such a generator often uses an excimer laser as a light source, and typically the radiation output from the laser is passed a radiation beam-scrambling illuminator to distribute the light intensity uniformly over the SLM surface. The illuminator includes a beam homogenizer, which is schematically illustrated in FIG. 1. The homogenizer consists of a lens system containing an array of lenses 1, each of which together with an imaging lens 2 distribute a respective transversely separated portion 3 of the laser beam 5 over the SLM surface 7 to thereby provide for a uniform integrated illumination of the SLM surface 7. The more lenses used in the array 3, the more uniform illumination of the SLM surface 7 is achieved.
One important limitation in this respect is the spatial coherence of the laser source. An excimer laser is known to have a temporal coherence length and a spatial coherence length, i.e. coherence length across the laser beam profile. Some lasers may even be spatially coherent over their entire beam width or at least over a major portion thereof. The temporal coherence length depends on the laser design and may be e.g. 0.15 mm. For books on coherence see Born and Wolf, Principles of Optics; Siegman, Lasers; and Goodman, Statistical Optics.
When the width D of the lenses in array 3 is smaller than the spatial coherence length ls of the laser source, light bundles from adjacent lenses 3 are obviously coherent, and as a consequence these light bundles may interfere with each other and produce an interference pattern in the illumination of the SLM surface 7 as schematically indicated by the xe2x80x9ctop hatxe2x80x9d light distribution with fringes to the right of SLM surface 7 in FIG. 1.
The spatial coherence length of the laser beam obviously puts a limitation on the number of lenses that can be used in the homogenizer, and thus on the quality of homogenization. The relative coherence length as a fraction of the beam diameter is an invariant of the beam when it is expanded.
Accordingly, it is an object of the present invention to provide a device for homogenizing the spatial intensity distribution of a spatially coherent radiation beam, which overcomes the above-mentioned problem of interference pattern or poor homogenization of the radiation beam.
This object, among others, is according to a first aspect of the invention attained by a device, which comprises a grating and a radiation splitting and directing arrangement. The grating is arranged in the propagation path of the spatially coherent radiation beam for diffracting the coherent beam and thus decreasing the coherence length of a diffracted radiation beam in a direction orthogonal to the propagation direction of the radiation beam relative to the width of the radiation beam in that orthogonal direction. The radiation splitting and directing arrangement is arranged in the propagation path of the diffracted radiation beam for splitting the diffracted radiation beam into spatially separated portions and for superimposing the spatially separated portions to thereby form a radiation beam having a homogenized spatial intensity distribution.
Preferably, the spatially separated portions have each a cross-sectional width, which is shorter than the original spatial coherence length of the radiation beam, but longer, preferably much longer, than the spatial coherence length of the diffracted radiation beam in the orthogonal direction, to thereby prevent adjacent portions from interfering with each other while being superimposed.
The coherent radiation beam may be temporally coherent and then have a temporal coherence length, which is shorter, or much shorter, then its spatial coherence length. This condition can in a practical case be created by expansion of the beam.
The grating may be a transmission or a reflection grating, and may in the latter case be arranged in Littrow configuration. Alternatively, if the radiation beam has a rectangular cross section, the grating may be arranged to magnify the radiation beam in one direction to obtain a diffracted radiation beam of a substantially quadratic cross section.
The splitting and directing arrangement may include one or several array of lenses, preferably cylindrical lenses, each of which focuses a respective one of the spatially separated portions, and a lens arrangement for imaging the spatially separated portions onto each other.
Further objects of the present invention are to provide a radiation beam conditioning device and an apparatus for high precision printing of a surface pattern on a photosensitive surface of a workpiece, particularly a photomask for semiconductor devices and displays, which make use of the device according to the first aspect of the invention. The lenses can be refractive, reflective or diffractive.
Thus, according to a second aspect of the present invention there is provided a radiation beam conditioning device for use in an apparatus for high precision printing or inspection of a surface pattern on a photosensitive surface of a workpiece, particularly a photomask for semiconductor devices and displays, using a spatially coherent radiation beam. The conditioning device comprises a radiation beam homogenizer according to the first aspect of the invention for homogenizing the spatial intensity distribution of the spatially coherent radiation beam.
Further, according to a third aspect of the invention there is provided an apparatus for high precision printing or inspection of a surface pattern on a photosensitive surface of a workpiece, particularly a photomask for semiconductor devices and displays. The apparatus comprises a source for emitting a spatially coherent radiation beam; a radiation beam conditioning device for shaping and homogenizing the spatial intensity distribution of the radiation beam; a spatial light modulator having multitude of modulating elements, illuminated by the conditioned radiation beam; and a projection system for creating an image of the spatial light modulator on the photosensitive surface of the workpiece. In the case of inspection there is an imaging capturing device such as a CCD, CID or MOS camera. The radiation beam-conditioning device includes the device according to the first aspect of the invention for homogenizing the spatial intensity distribution of the spatially coherent radiation beam.
Still further objects of the present invention are to provide a method for homogenizing the spatial intensity distribution of a spatially coherent radiation beam, which overcomes the problem of interference pattern or poor homogenization of the radiation beam, and to provide a method for high precision printing or inspection of a surface pattern on a photosensitive surface of a workpiece, particularly a photomask for semiconductor devices and displays, which makes use of the homogenizing method.
Thus, according to a fourth aspect of the present invention there is provided a method for homogenizing the spatial intensity distribution of a spatially coherent radiation beam, comprising the actions of: (i) diffracting the coherent beam and thus decreasing the coherence length of a diffracted radiation beam in a direction orthogonal to the propagation direction of the radiation beam relative to the width of the radiation beam in that orthogonal direction; (ii) splitting the diffracted radiation beam into spatially separated portions; and (iii) superimposing the spatially separated portions to thereby form a radiation beam having a homogenized spatial intensity distribution.
According to a fifth aspect of the invention there is provided a method for high precision printing or inspection of a surface pattern on a photosensitive surface of a workpiece, particularly a photomask for semiconductor devices and displays, which method comprises the actions of: (i) emitting a spatially coherent radiation beam; (ii) homogenizing the spatial intensity distribution of the radiation beam; (iii) illuminating a spatial light modulator having multitude of modulating elements with the homogenized radiation beam; and (iv) creating an image of the spatial light modulator on the photosensitive surface of the workpiece by means of a projection system. The step of homogenizing is performed in accordance with the fourth aspect of the invention.
Further, according to a sixth aspect of the invention there is provided a device for homogenizing the spatial intensity distribution of a spatially coherent radiation beam comprising a deflection device arranged in the propagation path of the spatially coherent radiation beam to deflect the coherent beam and thus decrease a spatial coherence length of a deflected radiation beam in a direction orthogonal to the propagation direction of the radiation beam relative to the width of the radiation beam in said orthogonal direction; and a radiation splitting and directing arrangement arranged in the propagation path of said deflected radiation beam to split the deflected radiation beam into spatially separated portions and to superimpose said spatially separated portions to thereby form a radiation beam having a homogenized spatial intensity distribution.
The deflecting device is preferably a segmented mirror, i.e. micro mirror array, or it may be realized by refractive optics.
Further, the deflecting device may be two-dimensional to reduce the spatial coherence length in two orthogonal directions across the beam simultaneously.
Further characteristics of the invention, and advantages thereof, will be evident from the detailed description of preferred embodiments of the present invention given hereinafter and the accompanying FIGS. 2-8, which are given by way of illustration only, and thus are not limitative of the present invention.